1. Field of the Invention
This invention relates to a bidirectional digital signal communication system and, more particularly, to a novel and highly-effective bidirectional digital signal communication system especially adapted for use in transmitting a digital data signal through a CATV (cable television) transmission line and similar communication links.
2. Description of the Prior Art
Generally, in a bidirectional digital signal communication system, a digital information or data signal is transmitted from a center unit (broadcasting station) through a "down" transmission line ("down-link") to each terminal unit, and each terminal unit transmits a digital data signal through an "up" transmission line ("up-link") to the center unit.
In the case of transmission through the up-link, in order to prevent deterioration of the S/N (signal-to-noise) ratio due to heat generated by a resistor in the last stage of the terminal unit, etc., the data signal and a pseudo-noise (PN) code are multiplied with each other. This produces a so-called "spread spectrum" encoding the data signal. The data multiplied by the PN code has a wide frequency spectrum so that it is less affected by noise during transmission than it would otherwise be. The data as encoded in the spread spectrum is transmitted in the form, for example, of a PSK-modulated signal (i.e., a signal modulated by phase shift keying).
In a bidirectional digital signal communication system using a spread spectrum for the transmission of data, it is necessary that the receiver of the center unit accurately synchronize the position (phase) of the code pattern of a received signal and the pulse repetition rate or frequency of the bit clock, both of which are transmitted from the terminal unit, with those of a reference signal in the receiver of the center unit.
However, the process of synchronization requires a predetermined time, and, if the synchronization process is carried out each time the polling of the terminal units is conducted by the center unit, the amount of data receivable from the terminal units per unit of time is substantially reduced, so that the transmission efficiency is lowered.